This invention relates to current limiting fuses, and in particular it relates to current limiting fuses having fusible elements of aluminum or aluminum alloys.
Current limiting fuses are well known in the art. One use for such a current limiting fuse is in a motor starter where the fuse is used in combination with an overload relay and/or a circuit breaker. The fuse limits large fault currents so that the overload relay is not damaged. Canadian Pat. No. 941,444-KRUZIC, issued Feb. 5, 1974, describes such a combination of current limiting fuse and overload relay. Another use for a current limiting fuse is to protect capacitors in a capacitor bank used, for example, to provide phase angle correction in an electrical system. The capacitors in a bank used for phase angle correction are usually in a series/parallel arrangement to provide the required correction at the necessary voltage. If a capacitor fails, all the capacitors in parallel with it will tend to discharge through the failed capacitor and this could involve a considerable amount of electrical energy. Each capacitor is therefore protected with a current limiting fuse.
A high voltage, current limiting fuse usually has a cylindrical casing of insulating material with a terminal cap or end cap of conducting metal closing each end. At least one fusible element or fuse element is connected between the end caps and the casing is filled with an arc-quenching material such as quartz sand to assist in extinguishing arcs which form along the fuse elements when it is subjected to fault currents. The fuse elements have areas of reduced cross-sectional areas at intervals along the length of the ribbon-like fuse elements to encourage arc formation and limit the fault current.
A current limiting fuse used in a motor starting circuit may have between perhaps 2 and 24 fuse elements extending in an uncoiled arrangement between end caps for a motor operating up to, for example, 5 kV (kilovolts). A high voltage, current limiting fuse for use in a capacitor bank may have, for example, a single helically coiled fuse element of perhaps 5 ft. to 8 ft. in length extending between the end caps. The fuse elements in the past have, with few exceptions, been of silver ribbon. It will be seen that fuses of these types use a considerable amount of silver. Silver is an expensive material and it is not always readily available. Many attempts have been made to use a less expensive material for a fuse element and to use a more readily available material. U.S. Pat. No. 4,150,354--NAMITOKOV et al, issued Apr. 17, 1979 describes a current limiting fuse with a fuse link of aluminum or aluminum alloy. Aluminum is a cheaper material than silver, it is more widely available, and it has reasonably good conductivity. These qualities make aluminum a reasonable alternative to silver. In this aforementioned United States patent the terminal contacts or end caps are of aluminum to prevent electrochemical corrosion where they connect to the fuse link and the disclosure indicates tested fuses have been operated at 660 V.
It is not always convenient or desirable to make the end caps of a cartridge fuse of aluminum in order to avoid joining aluminum with a different metal. However, it is difficult to join an aluminum fuse element to another material, such as copper for example, to make a reliable joint that will withstand repeated temperature cycling.
One object of the present invention is to provide a novel high voltage, current limiting fuse, having an aluminum fuse element joined to a copper terminal where the terminal is joined to the end cap.
It is another object of the invention to provide in a current limiting fuse, an aluminum fuse element having a novel arrangement of areas of reduced cross-section suitable for encouraging arc formation with overload currents.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.